<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input id="num1"/>
<select  id="fuhao">
   <option value="+">+</option>
   <option value="-">-</option>
   <option value="*">*</option>
   <option value="/">/</option>
</select>
<input id="num2"/>
<input type="button"  value="="  onclick="jiSuan()"/>
<input id="num3"/>
</body>
<script>
    function jiSuan() {
       if(fuhao.value=="+"){
           num3.value=parseInt(num1.value)+parseInt(num2.value);
       }else if(fuhao.value=="-"){
           num3.value=parseInt(num1.value)-parseInt(num2.value);
       }else if(fuhao.value=="*"){
           num3.value=parseInt(num1.value)*parseInt(num2.value);
       }else if(fuhao.value=="/"){
           num3.value=parseInt(num1.value)/parseInt(num2.value);
       }
    }
</script>
</html>